Hormonal responses to Exercise

 The purpose of this article is to briefly explain hormonal responses to different types of exercise and intensities.  Endocrinology, structure of hormones, and related physiological changes as well as adaptations can be quite complicated. For this reason, the explanation provided will be simplified, limited, and focused on the several major players so to speak. Results, adaptations, and general descriptions of the physiological parameters will be discussed without presenting structures of each and disease processes that may be associated with over or under production of such hormones. Rather, the focus will be on the major hormones involved to initiate and maintain certain types exercise and intensity. The basic responses and adaptations related to exercise intensity, duration and volume will be the primary focus of this explanation. 

 

The hypothalamus is a part of the brain that initiates the fight or flight response. During periods of stress, the hypothalamus sends signals to the adenohypophysis or anterior pituitary to secrete the hormones necessary to initiate and engage in exercise. The anterior pituitary produces seven different hormones. These hormones include follicle stimulating hormone (FSH), luteinizing hormone (LH), thyroid stimulating hormone (TSH), melanocyte stimulating hormone (MSH), adrenocorticotropic hormone (ACTH), prolactin, and growth hormone (GH). FSH as the name implies stimulates the follicle to develop in the ovaries prior to ovulation in females. This also brings about the production of estrogen to prepare the female body for ovulation and potentially support a child in the event of conception. Once ovulation takes place, the corpus luteum that remains releases progesterone in response to LH. Progesterone supports and development of the fetus if conception occurs. FSH in males stimulates spermatogenesis and production of seminal fluid. LH stimulates the Leydig cells of the testes to produce testosterone. Testosterone aids in development in secondary sex characteristics and promotes muscle hypertrophy. Contrary to many claims, testosterone while increasing lean body mass does not directly burn body fat. TSH stimulates the production of T4 and T3 from the thyroid gland. Thyroid hormones elevate metabolism in men and women. MSH stimulates melanocytes to produce melanin in response to sunlight and plays a role in vitamin D production. Vitamin D in turn plays a significant role in calcium absorption and bone remodeling. ACTH is a bit more complicated. It stimulates the adrenal cortex in a cascade of hormone production that plays a very significant role in exercise execution and adaptations. The adrenal cortex has three zones that produces different hormones essential to the fight or flight response. The outer zone is called the zona glomerulosa. This zone produces a mineralocorticoid called aldosterone which promotes sodium and fluid reabsorption into the bloodstream to increase blood pressure. The second zone is called the zona fasciculata which produces glucocorticoids called epinephrine or adrenaline and norepinephrine or noradrenaline. These hormones increase heart rate, contract blood vessels raising blood pressure, dilate airways, and liberate glucose into the blood via adipose tissue, glycogen stores, and liver gluconeogenesis. All of these are to get oxygen and nutrients to working muscles to facilitate continued muscle contraction. The inner zone is called zona reticularis which produces weak androgens in both men and women. This zone produces a substance called dehydroepiandrosterone (DHEA) which can be converted to testosterone in small amounts. As a side note, testosterone is responsible for sex drive in both men and women. GH is a peptide that has a profound significance during exercise and for adaptations to exercise. Epinephrine starts lipolysis and proliferation of energy substrates and GH continues it except for the effect on glycogen stores. GH continues fat metabolism, stimulates muscle growth by the assimilation of the contractile proteins actin and myosin, increases bone growth and bone mineral density. GH also stimulates the release of insulin-like growth factor (IGF-1) from the liver which allows sugar into the muscle and contributes to muscle fiber hypertrophy by promoting glycogen stores.  

Epinephrine starts the fat breakdown process and growth hormone continues it. Testosterone increases lean body mass which may in turn slightly increase the number of calories burned daily, but does not directly burn body fat. This claim is made very often by the testosterone booster industry. It is also suggested that cortisol lowers free testosterone levels. This notion should be proceeded with some skepticism. The only research that was found in an attempt to substantiate this claim injected subjects with cortisol before sleep. During REM sleep is when a person releases the most testosterone and growth hormone. Cortisol and testosterone use the same carrier proteins. When one injects cortisol and it is competing for the same carrier proteins, it is only logical that the subjects will have restless sleep patterns and lowered free testosterone levels in the morning. The research design appears to be used to achieve the results desired. Being open minded, a well-designed research study is greatly desired and would be appreciated. Unfortunately, the scientific method takes quite a beating in “research” these days. Everyone should be skeptical of research claims such as ketones are healthy, a certain food burns fat, everything that boosts metabolism or is essential for weight loss; but cannot cite any supporting well done research or even logic to support the premise. There are many claims and hoaxes in the fitness industry that are not supported by physiology or a shred of scientific evidence. 

 

Both aerobic exercise and resistance training have beneficial effects on health and hormonal responses. Aerobic exercise can be generally catabolic which simply means that the body is breaking down substrates to liberate energy for muscle contraction. Aerobic exercise done correctly will use a combination of carbohydrates and fatty acids for energy. Aerobic metabolism takes place at sixty to eighty percent of age predicted maximum heart rate (APMHR) and fifty to seventy percent of VO2 maximum. Aerobic exercise done correctly will utilize only fatty acids and carbohydrates, but not protein or muscle tissue if the individual is properly nourished. Intensity of aerobic exercise above eighty percent of APMHR and seventy percent of max VO2 will sacrifice muscle protein and glycogen stores for energy. Muscle takes too much time and effort to develop, so please do not sacrifice muscle by using inappropriate cardiovascular training intensity. The ability to maintain muscle during aerobic activity is supported by proper intensity and also appropriate caloric consumption as well as adequate amounts of macronutrients. The primary hormones involved in aerobic exercise are epinephrine and growth hormone produced significantly from twenty minutes and beyond during aerobic exercise. These hormones support the use of fatty acids and carbohydrates for energy. 

 

 Epinephrine and norepinephrine play a significant role in aerobic metabolism. As explained previously, epinephrine increases blood sugar levels and heart rate during exercise. Norepinephrine has many functions, but in this case the primary function is to restrict blood flow to the muscles not engaged in the exercise bout. Glucagon is a hormone produced by the alpha cells of the Islets of Langerhans located within the pancreas. Glucagon is an antagonist to insulin which liberates sugar into the blood during exercise. 

 

For safety, cardiovascular benefits, and body composition changes; aerobic exercise should be performed thirty to sixty minutes per session, one to two sessions per day, and three to five days per week at the intensity presented previously. Adaptations to adequate aerobic activity include lower resting and exercise heart rate, decreased blood pressure both systolic and diastolic, caloric expenditure, increased insulin sensitivity, improved oxygen extraction of skeletal muscles, decreased resting cortisol levels, endorphins, and decreased body fat percentage if accompanied by an appropriate nutrition program and food selection. 

 

Resistance training may seem simple to a novice and many joke that we pick up weights and put them down. After developing resistance training, cardiovascular, and nutrition programs for over thirty years encompassing everyone from monitored cardiac rehabilitation patients to athletes including bodybuilders, wrestlers, and endurance competitors; I can assure everyone that it can be far more complex than many people realize. Well-designed resistance training will vary considerably in exercises chosen, intensity, volume, sets, repetitions, and rest intervals. All these variables will affect how muscles are sculpted, adapt, and hormonal response. 

 

During resistance training, there is an immediate increase epinephrine and norepinephrine. These hormones elevate blood glucose levels as well as increase in force production, muscle contraction rate, and energy production of energy substrates via synthesis of adenosine triphosphate (ATP) which is the energy supply of cells. ATP is the fuel directly responsible for muscle contraction. These hormones increase in anticipation of resistance training to different degrees based on the individual.  This response is produced in anticipation of the challenging exercise to be accomplished. The increase in blood glucose is not generally followed by an increase in the hormone insulin unless protein and/or carbohydrate supplementation is ingested before exercise. Instead, the increase in glucose uptake by the skeletal muscle is achieved by an increase in glucose transporters within cells. This glucose uptake is accompanied by an increase in glucose metabolism within the muscle cell. For this reason, regular resistance training increases insulin sensitivity. 

 

Volume of resistance training exercise ca be calculated by analyzing the number of sets, repetitions, and resistance or weight. A repetition base of fifteen or greater is referred to as an endurance or toning protocol or regimen. A repetition base of eight to twelve is called a muscle hypertrophy or muscle building protocol and a repetition base of six or less are for strength and power development. A low volume circuit resistance training protocol as compared to a high volume periodized resistance training scheme yield different hormonal responses and adaptations. The higher volume resistance protocol produces higher testosterone, insulin like growth factor (IGF-!)  values, and lower cortisol levels when compared to circuit lower volume training. Greater increases in strength, power, and speed are associated with higher volume training. 

 

The number of sets utilized has been investigated using all three protocols of repetitions being endurance, muscle hypertrophy, and strength. In strength protocols, the number of sets does not appear to affect hormonal responses. Muscular hypertrophy and endurance protocols produced an increase in cortisol and growth hormone when using four sets rather than two sets. The different number of sets does not display a significant difference in testosterone levels. In summary, the high-volume training elevates testosterone, cortisol, and growth hormone. High volume has the largest hormonal response. 

 

As an exercise physiologist, my belief at one time was that it was very important to take one or multiple sets to absolute failure. Studies have shown that failure and non-failure training resulted in similar gains in one repetition maximum strength, muscle power output, and maximal number of repetitions. After a specified timeframe, maximum strength and power peaked for both types of groups.  It appears the non-failure group of people showed greater increases in strength, power, resting testosterone, and lower cortisol levels than those who went to failure. The failure group also demonstrated a decrease in the hormone IGF-1. The lesson learned is that muscular failure is not as important as once believed for muscle strength, power, and hormone response. Taking each set to failure may actually lead to overtraining and diminished hormonal and muscle power adaptations. 

 

Conventional resistance training has a sequential concentric and eccentric action. Concentric action produces, more growth hormone compared to eccentric action. This was the case when the same absolute load was used. However, when a relative load was used similar testosterone and growth hormone levels were produced. For best hormonal response, a program that varies concentric and eccentric actions should be incorporated for best results. 

 

Forced repetitions are a method for adding intensity to a resistance training. Forced repetitions requires the individual to perform repetitions beyond the point of failure.  This requires a personal trainer or training partner. Maximal repetitions are taking a set to failure. Levels of testosterone, growth hormone, and cortisol has shown to increase in a similar matter for both training scenarios.  However, testosterone level increases were significantly greater using forced repetitions when performed by an experienced weightlifter than by a novice or less experienced lifter. The use of forced repetitions and training until failure should be used in moderation especially for less experienced lifters. Always taking sets until failure may have negative effects on strength, power, and hormonal response. It does appear that the more experienced and trained the individual, the greater the muscle building hormonal response when forced repetitions are periodically utilized. 

 

The amount of rest taken between sets can be used to ramp up or lower training intensity. The rest period can be as high as five minutes and as low as no time between sets accomplished through circuit training and depending on individual goals. Different rest intervals have not shown to demonstrate a significant difference in hormone response. 

 

Resistance training has shown a significant effect on acute hormone levels after training. These responses have a huge role in immediate tissue remodeling and growth. It also has a profound effect on long term gains in strength, power, and muscle hypertrophy. It is recommended that people engage in resistance a minimum of two to three days per week. The greatest responses for optimal results are accomplished by incorporating large muscle mass using compound movements, high volume training sessions, and moderate to high intensity. 

 

 

Hormonal Response and Adaptation to Resistance Exercise and Training; Kraemer et al, Sports Med. 2005 35(4) 338-361 

Volek JS, Kraemer WJ, Bush JA, et al. Testosterone and cortisol in relationship to dietary nutrients and resistance exercise. J 8. Hakkinen K, Pakarinen A. Acute hormonal responses to heavy ¨ Appl Physio 1997; 8: 49-54